Cyclic behavior of endplate connections to tubular columns with novel slip-critical blind bolts

This paper presents an experimental investigation on beam-to-column blind bolted endplate connections using innovative Slip-Critical Blind Bolts (SCBB). Common bolts cannot be adopted in hollow section columns due to the impossibility of reaching the inner side of the columns and the blind bolt is a good solution. A split-type spacer was introduced into the blind bolt, enabling almost the same pre-tightening mechanism as traditional high strength bolt. This special detailing provides possibility of sufficient pre-tightening force required by major codes and makes the blind bolt slip-critical. A specially-made instrument is developed to install the blind bolt expediently. A series of experiments was conducted to investigate the pre-tightening force and mechanical behavior of the individual blind bolts under tension, shear and combined tension and shear, and the SCBB demonstrated desirable fastening performance. Eight full-scale beam-to-column cruciform extended endplate connections using the SCBB were subsequently tested under cyclic loading. Three types of stiffening strategies were adopted for the SHS columns, i.e. inner diaphragms, thickened column wall, and filled concrete. For all the cases, the SCBB performed very well and exhibited almost identical behavior to the traditional high strength slip-critical bolts. Two failure modes of the connections, namely, beam bucking (mode 1) and column damage (mode 2), were generally observed and were found to be influenced by the column stiffening strategies. The stiffening measures were also found to greatly influence the stiffness, strength, ductility, and energy dissipation of the connections. The test result generally showed that a properly designed SCBB connection could exhibit very satisfactory cyclic performance, demonstrating a great potential of such connection for seismic resistance applications.